1. Field
The presently disclosed subject matter relates to a friction plate for a wet-type multiplate clutch useful as a clutch, brake or the like in an automatic transmission.
2. Description of the Related Art
FIG. 1 is a cross-sectional view illustrating the fundamental construction of a wet-type multiplate clutch 10. FIG. 1 shows a clutch casing 21, a counterpart hub 22 to which a rotary torque is transmitted, a spline groove 23 formed on the clutch casing 21, a spline groove 24 formed on the hub 22, a piston 25 for pressing separator plates 30 and friction plates 40 against a backing plate 26, a snap ring 27 supporting the backing plate 26, and a sealing ring 28 for the piston 25. The separator plates 30 are maintained in fitting engagement with the spline groove 23, while the friction plates 40 are maintained in fitting engagement with the spline groove 24.
In recent years, there is an ever-increasing demand for improvements in the fuel economy of automobiles. Keeping in step with this trend, there is an outstanding demand for a further reduction in drag toque between friction plates and separator plates during idling of a clutch in an automatic transmission.
A conventional clutch is equipped with friction plates provided with friction linings, each of which has one or more oil grooves having closed end portions to separate the friction plates from their associated separator plates during idling of the clutch and also has one or more oil passages extending radially through the friction lining to feed lube oil onto a friction surface for the prevention of seizure during engagement of the clutch (see, for example, JP-A-11-141570 and JP-A-2005-076759).
To improve the shift response in an attempt to make not only an improvement in fuel economy but also improvements in engine performance, the clearances between friction plates and their associated separator plates have increasingly become smaller recently, tending to result in a greater drag torque due to intervening oil films during idling.
With such conventional friction plates, sufficient drainage of lube oil is sometimes not feasible on their friction surfaces, thereby failing to satisfactorily meet the demand for a further reduction in drag torque.